Tunnel field-effect transistor with reduced trap-assisted tunneling leakage

ABSTRACT

The current disclosure describes a tunnel FET device including a P-I-N heterojunction structure. A high-K dielectric layer and a metal gate wrap around the intrinsic channel layer with an interlayer positioned between high-K dielectric layer and the intrinsic channel layer of the P-I-N heterojunction. The interlayer prevents charge carriers from reaching the interface with high-K dielectric layer under the trap-assisted tunneling effect and reduces OFF state leakage.

BACKGROUND

Metal-oxide-semiconductor (MOS) field-effect transistors (FET) have beena dominating technology for integrated circuits. A MOSFET can work inthree regions, depending on gate voltage V_(g) and source-drain voltageV_(ds). These three regions include linear, saturation, andsub-threshold regions. The sub-threshold region is a region wherein gatevoltage V_(g) is smaller than threshold voltage V_(t). The sub-thresholdswing represents the easiness of switching the transistor current offand is an important factor in determining the speed and power of a MOSdevice. The sub-threshold swing can be expressed as a function ofm*kT/q, wherein m is a parameter related to capacitance. Thesub-threshold swing of conventional MOS devices has a limit of about 60mV/decade (kT/q) at room temperature, which in turn sets a limit forfurther scaling of operation voltage VDD and threshold voltage Vt. Thislimitation is due to the drift-diffusion transport mechanism ofcarriers. For this reason, existing MOS devices typically cannot switchfaster than 60 mV/decade at room temperatures. The 60 mV/decadesub-threshold swing limit also applies to FinFET or ultra-thin bodyMOSFET on silicon-on-insulator (SOI) devices. Therefore, with bettergate control over the channel, a newer ultra-thin body MOSFET on SOI ora finFET can achieve a sub-threshold swing close to, but not below, thelimit of 60 mV/decade. With such a limitation, faster switching at lowoperation voltages for future nanometer devices is challenging toachieve.

The tunnel field-effect transistor (TFET) is a newer type of transistor.TFETs switch by modulating quantum tunneling through a barrier. Becauseof this, TFETs are not limited by the thermal Maxwell-Boltzmann tail ofcarriers, which limits MOSFET subthreshold swing to about 60 mV/decadeof current at room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. In thedrawings, identical reference numbers identify similar elements or actsunless the context indicates otherwise. The sizes and relative positionsof elements in the drawings are not necessarily drawn to scale. In fact,the dimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1-5 illustrate example TFET structures according to embodiments ofthe disclosure;

FIG. 6 is an example flow diagram of an example fabricating processaccording to embodiments of the disclosure; and

FIGS. 7A-7H illustrate a wafer at various stages of fabrication throughthe example process of FIG. 6.

DETAILED DESCRIPTION

The disclosed techniques are directed to tunnel field-effect transistors(TFET) that have an interlayer of semiconductor material positionedbetween a channel layer and a gate structure. A TFET transistor includesa source of a first conductivity type, a drain of a second conductivitytype, an intrinsic or unintentionally doped channel, a gate adjacent tothe channel, which includes a gate electrode and a high-K gatedielectric, and an interlayer positioned between the high-K gatedielectric and the channel. In an embodiment, a material of theinterlayer includes a wider bandgap than a material of the channel. Theexistence of the additional interlayer between the channel and thehigh-K gate dielectric increases the physical distance for chargecarriers to reach the interface of the semiconductor/dielectric junctionfrom the channel layer under the trap-assisted tunneling (TAT) effectand also increases the energy barrier for TAT tunneling. At the sametime, the major band to band tunneling (BTBT) effect from the source tothe drain through the channel is not affected by the additionalinterlayer because the interlayer is not in the route of the BTBT. Withthe TAT tunneling effect being suppressed, the subthreshold swing (SS)of the TFET is decreased and the OFF-state current, i.e., the leakagecurrent, is reduced.

An example core-shell TFET structure includes a substrate and a baselayer of a first III-V compound semiconductor material (III-V material),e.g., InAs, over the substrate. A III-V compound semiconductor materialrefers to a chemical compound with at least one group III element (thechemical elements in group 3 of the periodic table, also called theboron group) and at least one group V element (chemical elements ingroup 5 of the periodic table, also called the nitrogen family). Ananowire core structure is formed over the base layer. The corestructure includes a drain structure of the first III-V material InAsover the base layer, a barrier layer of a second III-V material, e.g.,GaAs, over the drain structure, and a source structure of a third III-Vmaterial, e.g., GaSb, over the barrier layer. A channel layer of anIII-V material, e.g., InAs, wraps around and contacts both the sourcestructure and the drain structure. An interlayer of a semiconductormaterial, e.g., GaAs or InP, wraps around the channel layer. A gatestructure wraps around the interlayer. The interlayer of GaAs or InPincludes a large conduction band offset ΔE_(c), with respect to thechannel layer of InAs. The drain structure is doped as a firstconductivity type, e.g., N-type, and the source structure is doped as asecond different conductivity type, e.g., P-type. The channel layer isintrinsic or unintentionally doped (UID), e.g., as N-type.

An example axial vertical TFET structure includes a substrate and a baselayer of a first III-V material, e.g., P doped GaN, over the substrate.A nanowire source structure of the first III-V material, P-doped GaN, isformed over the base layer. A nanowire channel layer of a second III-Vmaterial, e.g., intrinsic or unintentionally doped InN or InGaN, isformed over the source structure. A nanowire drain structure of a III-Vmaterial, e.g., N-doped GaN, is formed over the channel layer. Aninterlayer of a semiconductor material wraps around at least the channellayer. A material of the interlayer layer includes a large conductionband offset (ΔE_(c)) with respect to the channel layer of InN/InGaN. Forexample, the interlayer is AlN. A gate structure wraps around theinterlayer layer. The interlayer separates the gate structure from thechannel layer. In the case that the gate structure is also adjacent tothe source structure or the drain structure, the interlayer also extendtoward the source structure or the drain structure to separate the gatestructure from them. A material of the interlayer layer includes a largeconduction band offset (ΔE_(c)) with respect to a material of thechannel layer and the relevant source or drain structure if adjacent tothe gate structure.

Further, in the cases that the interlayer also contacts one or more ofthe source or the drain, a material of the interlayer includes a widerbandgap than the channel layer with respect to the one or more of thesource or the drain. That is, the interlayer includes a larger tunnelingbarrier than the channel layer such that the BTBT tunneling occursthrough the channel layer instead of the interlayer.

In the above examples, the core-shell TFET includes a lateral tunnelingand the axial TFET includes a vertical tunneling. In other examples, acore-shell TFET having a vertical BTBT or an axial TFET having a lateralBTBT are also possible and included in the disclosure. The interlayerbetween the gate and the channel layer may also be used in TFET devicesthat include both vertical and lateral tunneling components.

For example, an example lateral axial TFET structure includes asubstrate and a source region, a channel region and a drain region overthe substrate. The channel region is positioned laterally between thesource region and the drain region. A gate structure is positionedadjacent to the channel region. An interlayer is positioned between thegate structure and the channel region. A material of the interlayerincludes a large conduction band offset (ΔE_(c)) with respect to amaterial of the channel region.

The existence of the wide bandgap semiconductor interlayer between thesemiconductor channel and gate dielectric layer moves the interfacetraps further away from the tunnel junction both spatially andenergetically. The TAT tunneling barrier is increased and the energyrequired for the thermal emission out of a trap is also increased. Thedesired main band-to-band tunneling “BTBT” is not affected by theadditional interlayer because the interlayer is not in the route of theBTBT. Accordingly, the tunneling length (spatial and energetic) for theBTBT tunneling and the tunneling length for the TAT tunneling aredecoupled from one another and are adjusted separately. As the TATtunneling effects are suppressed, the Off state current is reduced andthe subthreshold swing (“SS”) is decreased, i.e., improved.

The disclosure herein provides many different embodiments, or examples,for implementing different features of the described subject matter.Specific examples of components and arrangements are described below tosimplify the present description. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with electronic componentsand fabrication techniques have not been described in detail to avoidunnecessarily obscuring the descriptions of the embodiments of thepresent disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

Gate all around (GAA) transistor structures may be patterned by anysuitable method. For example, the structures may be patterned using oneor more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the GAA structure.

The following description refers to a transistor as an example of asemiconductor structure to which the present description applies;however, the present description is not limited in applicability totransistors. For example, the follow description applies to other typesof semiconductor structures that are not transistors where the reducedTAT effects are desirable.

FIG. 1 illustrates an example axial TFET device 100. As shown in FIG. 1,the TFET device 100 includes a substrate 110, e.g., a silicon substrate,and a base layer 120 of a first semiconductor material, e.g., galliumnitride “GaN”, over the substrate. In some embodiments, a nucleationlayer 122, e.g., of aluminum nitride “AlN”, is disposed between thesubstrate 110 and the base layer 120. The base layer 120 is doped as afirst conductivity type, e.g., P-type. A vertical heterojunction stack130 is formed over the base layer 120. For example, the vertical stack130 is a stack of nanowire structures. The nanowire stack 130 includes ananowire source layer 132 of the first conduction type, e.g., P-type, anintrinsic or unintentionally doped (UID) nanowire channel layer 134, anda nanowire drain layer 136 of a second conductivity type, e.g., N-type.A gate structure 140 is formed adjacent to the channel layer 134. In anembodiment, as shown in FIG. 1, the gate structure 140 wraps around thechannel layer 134. The gate structure 140 includes a gate dielectriclayer 142, e.g., of a high-K dielectric material, and a gate electrode144.

An interlayer 150 is formed between the gate structure 140 and thechannel layer 134, or specifically between the gate dielectric 142 andthe channel layer 134. In an embodiment, the interlayer 150 includes asemiconductor material that has a large conduction band offset (orvalence band offset) with respect to the channel layer 134 such that theconduction band energy of the interlayer 150 is much larger than theconduction band energy of the channel layer 134. Resultantly, the energybarrier is increased for the tunneling to the interface with the gatedielectric layer 142, and the TAT tunneling current is thus decreased.For example, in case the channel layer 134 is InN or InGaN, theinterlayer 150 is AlN.

In an embodiment, as shown in FIG. 1, the gate structure 140 alsolaterally adjacent to or overlaps at least partially one or more of thesource layer 132 or the drain layer 136 and the interlayer 150 extendsbetween the gate structure 140 and the adjacent source layer 132 and thedrain layer 136. As such the source layer 132 and the drain layer 136are also separated from the gate structure 140 by the interlayer 150,which prevents the charge carriers trapped in the source layer 132 orthe drain layer 136 from reaching the interface of the gate dielectriclayer 142. In an embodiment, the interlayer 150 has a large conductionband offset with respect to the source layer 132 and the drain layer 136such that the TAT tunneling current, i.e., leakage, from the sourcelayer 132 or the drain layer 136 to the gate dielectric layer 142 can bereduced. Further, the interlayer 150 has a larger tunneling barrier thanthe channel layer 134. For example, the interlayer 150 has a largerenergy offset or bandgap, i.e., between the valence band on one side ofthe junction and the conduction band on the other side of the junction,with respect to the source layer 132 than the channel layer 134 suchthat it is ensured that the band-to-band tunneling BTBT occurs throughthe channel layer 134 instead of the interlayer 150. That is, theinterlayer 150 has a wider bandgap than the channel layer 134. Asdescribed above, AlN is a suitable material for the interlayer 150because it has valence band offset and conduction band offset againstboth InN and GaN and it has a wider bandgap than InN.

Further because the interlayer 150 includes a large conduction bandoffset with respect to the channel layer 134, the electronunder the TATtunneling effects needs extra energy to reach the interface between theinterlayer 150 and the gate dielectric layer 142. In other words, in thecase that the conduction band energy of the interlayer 150 is muchlarger than the trap energy under the TAT effect, the TAT current issubstantially reduced under the algorithm below:

e_(n) ∝ e^(−ΔE), andΔE = E_(c) − E_(t,)

where e_(n) is electron density, E_(t) is the energy level of theinterface traps and Ec is the conduction band energy, e.g., theconduction band offset between the interlayer 150 and the channel layer134.

In an embodiment, to maintain the gate control on the BTBT tunnelingthrough the channel layer 134, the thickness of the interlayer 150 iscontrolled to be thin. For example, in an embodiment, the interlayer 150includes a thickness ranging from about 1 nm to about 5 nm.

In an embodiment, the source layer 132 includes a thickness ranging fromabout 5 nm to about 30 nm, the channel layer 134 includes a thicknessranging from about 2 nm to about 20 nm, and the drain layer 136 includesa thickness ranging from about 5 nm to about 50 nm.

The example device 100 also includes interconnection/contact structures160 and inter-level dielectrics 170.

FIG. 2 shows another example device 200. The device 200 is similar tothe device 100 of FIG. 1 except that in the vertical stack ofsemiconductor layers 230, the channel layer 234 is formed over thesource layer 232 and also adjacent to the sidewall 232S of the sourcelayer 232. The drain layer 236 is formed over the channel layer 234.

As such, in the device 200, the BTBT tunneling includes both verticalcomponents and lateral components, as illustratively shown with thearrows. For the lateral components of the BTBT tunneling, as the chargecarrier movement direction is in parallel with the gate electricalfield, the gate control of the BTBT current is improved. As theinterlayer 150 of AlN includes a wider bandgap and larger tunnelingbarrier than the channel layer 234, the BTBT tunneling does not gothrough the interlayer 150.

In the devices 100, 200, the N-type TFETs of P-doped GaN source,intrinsic or UID InN channel, and N-doped GaN drain are used asillustrative examples, which does not limit the scope of the disclosure.An interlayer 150 may also be used in P-type TFETs and/or TFETs withother combinations of semiconductor materials. For example, in a P-typeTFET, the source layer may be N-doped indium arsenide InAs, the channellayer may be intrinsic (or UID) InAs, and the drain layer may be P-dopedgallium antimonide GaSb. The interlayer 150 may be gallium arsenideGaAs. In another P-type TFET, the source layer may be N-doped InAs, thechannel layer may be intrinsic (or UID) InAs, and the drain layer may beP-doped silicon. The interlayer may be GaAs or indium phosphide InP.Other combinations of semiconductor materials are also possible andincluded in the disclosure.

FIG. 3 shows an example core-shell TFET device 300. The device 300includes a substrate 310, e.g., a silicon substrate, and a base layer320 of a first semiconductor material, e.g., InAs, over the substrate310. The base layer 320 is doped as a first conductivity type, e.g.,N-type. A vertical heterojunction core stack 330 is formed over the baselayer 320. For example, the vertical core stack 330 includes nanowirestructures. The core stack 330 includes a drain layer 332 of the samesemiconductor material as the base layer 320 and doped as the firstconduction type, e.g., N-type, an intrinsic or unintentionally doped UIDbarrier layer 334 of a second semiconductor material, e.g., GaAs, and asource layer 336 of a third semiconductor material, e.g., GaSb, anddoped as a second conductivity type, e.g., P-type. A channel layer 338is formed adjacent to and contacting both the drain layer 332 and thesource layer 336. The channel layer 338 includes a fourth semiconductormaterial and is intrinsic or UID. In an example, the fourthsemiconductor material includes intrinsic InAs. The InAs material of thechannel layer 338 is different from the InAs material of the drain layer332 in that the InAs of the channel layer is intrinsic or UID, and theUID InAs of the channel layer 338 includes a smaller dopingconcentration than the N-doped InAs in the drain layer 332. A gatestructure 340 is formed adjacent to the channel layer 134. In anembodiment, as shown in FIG. 3, the channel layer 338 is shell-shapedand wraps around the drain layer 332 and the source layer 336, and thegate structure 340 wraps around the channel layer 338. The gatestructure 340 includes a gate dielectric layer 342, e.g., of a high-Kdielectric material, and a gate electrode 344.

In an embodiment, the barrier layer 334 include a larger tunnelingbarrier than the channel layer 338 with respect to the source layer 336so that the BTBT tunneling occurs through the channel layer 338 insteadof the barrier layer 334. For example, the barrier layer 334 may includea wider bandgap than the channel layer 338 and may include at least oneof a larger conduction band offset or a larger valence band offset withrespect to the source layer 336 than the channel layer 338. Theexistence of the barrier layer 334 may reduce leakage current at the OFFstate of the gate 340. At the ON state of the gate 340, the gate voltagefield is in parallel with the BTBT tunneling from the source layer 336to the channel layer 338, which improves gate control of the BTBTtunneling.

An interlayer 350 is formed between the gate structure 340 (orspecifically the gate dielectric 342) and the channel layer 338. Morespecifically, the interlayer 350 completely intervenes between the gatedielectric layer 342 and the channel layer 338 such that a trappedcharge carrier, i.e., an electron or a hole, in the channel layer 338cannot reach the gate dielectric layer 342 without going through theinterlayer 350. The interlayer 350 may completely overlap or cover thegate structure 340 in the direction between the gate structure 340 andthe core stack 330 or completely covers/overlaps the channel layer 338in the direction between the gate structure 340 and the core stack 330.In an embodiment, the interlayer 350 includes a semiconductor materialthat has a large conduction band offset with respect to the channellayer 338 such that the conduction band energy of the interlayer 350 ismuch larger than the trap energy through the channel layer 338 and theTAT current is decreased. For example, in the case that the channellayer 338 is InAs, the interlayer 350 is GaAs or indium phosphide InP.

The core-shell TFET 300 also includes contact structures 360 andinter-level dielectric 370.

FIG. 4 shows another example core-shell TFET 400. The core-shell TFET400 is similar to the core-shell TFET 300 of FIG. 3 except that the TFET400 includes an inner gate structure 440I and an outer gate structure440O. The inner gate structure 440I is inside a ring-shapedsemiconductor stack 430 and the outer gate structure 440O is outside thering-shaped semiconductor stack 430. Each of the inner gate structure4401 and the outer gate structure 440O includes a gate dielectric and agate electrode.

The semiconductor stack 430 includes a drain layer 432, a barrier layer434 and a source layer 436. Two channels layers 438O and 438I arepositioned adjacent to and contacting both the source layer 436 and thedrain layer 432 from outer sidewalls thereof (FIG. 4 shows only a singleouter side wall 430O as an illustrative example) and inner sidewallsthereof (FIG. 4 shows only a single inner side wall 430I as anillustrative example), respectively. Specifically, in an embodiment, theinner channel layer 438I surround both the source layer 436 and thedrain layer 432 by being wrapped around by the source layer 436 and thedrain layer 432. The outer channel layer 438O surrounds both the sourcelayer 436 and the drain layer 432 by wrapping around both the sourcelayer 436 and the drain layer 432.

An outer interlayer 450O is positioned between the outer gate structure440O and the outer channel layer 438O. An inner interlayer 450I ispositioned between the inner gate structure 440I and the inner channellayer 438I. In an embodiment, the outer interlayer 450O includes asemiconductor material that has a large conduction band offset withrespect to the outer channel layer 438O such that the conduction bandenergy of the outer interlayer 450 is much larger than the trap energythrough the outer channel layer 438O and the TAT current is decreased.In an embodiment, the inner interlayer 450I includes a semiconductormaterial that has a large conduction band offset with respect to theinner channel layer 438I such that the conduction band energy of theinner interlayer 450I is much larger than the trap energy through theinner channel layer 438I and the TAT current is decreased. For example,in the case that the inner channel layer 438I and the outer channellayer 438O are InAs, the inner interlayer 450I and the outer interlayer450O are GaAs or InP.

The example TFETS 100, 200, 300 and 400 all are shown with verticalstacks of semiconductor layers of drain layers 132, 232, 332, 432 andsource layers 136, 236, 336, 436, for illustrative purposes. Thedisclosure is not limited by such vertical semiconductor stacks. Forexample, FIG. 5 shows a lateral TFET 500. The TFET 500 includes asubstrate 510, e.g., a silicon substrate and an insulation layer, e.g.,a buried oxide (BOX) layer 520 over the substrate. A source region 532,a channel region 534 and a drain region 536 are positioned in a lateralorientation over the BOX layer 520. A gate structure 540 is positionedadjacent to the channel region 534. Specifically, the gate structure 540may be positioned over the channel region 534 or may wrap around thechannel region 534. The gate structure 540 includes a gate dielectric542 and a gate electrode 544. In an embodiment, the gate structure 540may also be adjacent to one or more of the source region 532 or thedrain region 536.

In an example N-type TFET embodiment, the source region 532 is P-dopedGaSb, the channel region 534 is intrinsic or unintentionally N-dopedInAs (N⁻) and the drain region 536 is N-doped InAs (N⁺). The N-dopedrain region 536 includes a higher doping concentration than theunintentionally N-doped channel region 534.

An interlayer 550 is positioned between the channel region 534 and thegate dielectric layer 542. In an embodiment, as shown in FIG. 5, theinterlayer 550 is also positioned between the gate dielectric 542 andone or more of the source region 532 or the drain region 536 layer. Inan embodiment, the interlayer 550 includes a semiconductor material thathas a large conduction band offset with respect to each of the channelregion 534, the source region 532 and the drain region 536 such that theconduction band energy of the interlayer 550 is much larger than thetrap energy through each of the channel region 534, the source region532 and the drain region 536. As a consequence, the TAT current isdecreased.

The substrate 110, 210, 310, 410, 510 is a silicon substrate or otherelement semiconductor like germanium or compound semiconductors like assilicon carbide, gallium arsenide, indium arsenide, and/or sapphire.Further, the substrate may also include a silicon-on-insulator (SOI)structure. The substrate may include an epitaxial layer and/or may bestrained for performance enhancement. The substrate may also includevarious doping configurations depending on design requirements as isknown in the art such as P-type substrate and/or N-type substrate andvarious doped regions such as P-wells and/or N-wells.

Further the substrate 110, 210, 310, 410, 510 may include variousinsulation bodies like shallow trench insulation (STI), deep trenchinsulation (DTI) and various local field oxide regions that separateactive areas/surfaces of device, e.g., the TFET devices.

The gate structures 140, 240, 340, 440 (440O, 440I), 540 are replacementgates of metal or other conductive materials. The following descriptionlists examples of materials for the gate structures. The gate electrodes144, 244, 344, 544 include a conductive material, e.g., a metal or ametal compound. Suitable metal materials for the gate electrode includeruthenium, palladium, platinum, tungsten, cobalt, nickel, and/orconductive metal oxides and other suitable P-type metal materials andinclude hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta),aluminum (Al), aluminides and/or conductive metal carbides (e.g.,hafnium carbide, zirconium carbide, titanium carbide, and aluminumcarbide), and other suitable materials for N-type metal materials. Insome examples, the gate electrode the gate structures include a workfunction layer tuned to have a proper work function for enhancedperformance of the field effect transistor devices. For example,suitable N-type work function metals include Ta, TiAl, TiAlN, TaCN,other N-type work function metals, or a combination thereof; andsuitable P-type work function metal materials include TiN, TaN, otherP-type work function metals, or combination thereof. In some examples, aconductive layer, such as an aluminum layer, a copper layer, a cobaltlayer or a tungsten layer is formed over the work function layer suchthat the gate electrodes of the gate structures include a work functionlayer disposed over the gate dielectric layers 142, 242, 342, 542 and aconductive layer disposed over the work function layer and below a gatecap (not shown for simplicity). In an example, the gate electrodes ofthe gate structures each a thickness ranging from about 5 nm to about 40nm depending on design requirements.

In example embodiments, the gate dielectric layer 142, 242, 342, 542include an interfacial silicon oxide layer (not separately shown forsimplicity), e.g., a thermal or chemical oxide having a thicknessranging from about 5 to about 10 angstrom (Å). In example embodiments,the gate dielectric layer 142, 242, 342, 542 further includes a highdielectric constant (high-K) dielectric material selected from one ormore of hafnium oxide (HfO2), hafnium silicon oxide (HfSiO), hafniumsilicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafniumtitanium oxide (HMO), hafnium zirconium oxide (HfZrO), combinationsthereof, and/or other suitable materials. A high K dielectric material,in some applications, includes a dielectric constant (K) value largerthan 6. Depending on design requirements, a dielectric material of adielectric contact (K) value of 7 or higher is used. The high-Kdielectric layer may be formed by atomic layer deposition (ALD) or othersuitable technique. In accordance with embodiments described herein, thehigh-K dielectric layer of the gate dielectric layer includes athickness ranging from about 10 to about 30 angstrom (Å) or othersuitable thickness. Other dielectric materials can also be used for thegate dielectric layer 142, 242, 342, 542, e.g., MgCaO or Al2O3.

In example embodiments, the inter-level dielectric layer 170, 270, 370,470, 570 is silicon oxide or a low-K dielectric material. A low-Kdielectric material includes as silicon oxynitride, silicon nitride(Si3N4), silicon monoxide (SiO), silicon oxycarbide (SiOC), vacuum, andother dielectrics or other suitable materials.

FIG. 6 shows an example fabrication process 600, which can be used tomake the example TFET 300 of FIG. 3 or other devices. In the descriptionherein, the example TFET 300 is used as an example to illustrate theexample process 600. FIGS. 7A-7H show various stage of a wafer 700 inmaking the example TFET 300.

Referring to FIG. 6, with reference also FIG. 7A, in example operation610, a wafer 700 is received. The wafer 700 includes a substrate 710 ofa silicon substrate in a crystalline structure, e.g., the Si(111) orSi(001), a base layer 720 of InAs over the substrate 710 and surroundedby STI 712. A mask layer 714 of a dielectric material, e.g., siliconoxide, silicon nitride or another low-K dielectric material, is formedover the base layer 720. The Base layer 720 is doped with a firstconductivity type. The base layer 720 of InAs is doped as N-type bysupply of additional Si or Ge containing precursors, or is doped asP-type, e.g., by using Mg containing precursors. Other suitable dopingprocedures, e.g., ion implantation of Si, Ge impurities for N-type or Mgimpurities for P-type, are also possible and included in the disclosure.For illustrative purposes, the base layer 720 of InAs is formed withN-type impurity.

The thickness of the base layer 720 is selected based on device designand structural strength considerations, e.g., aspect ratio. To provide asolid base for the subsequent vertical nanowire stack growth over thebase layer 720, the InAs base layer 720 is sufficiently thick to providelow dislocation density, e.g., smaller than 10⁸ cm⁻². In an embodiment,the thickness of the base layer 720 is within a range of about 50 nm toabout 200 nm.

In an example, the base layer 720 of InAs nanowire is formed on thesubstrate 710 using metalorganic chemical vapor deposition (MOCVD) ormolecular beam epitaxy (MBE) at temperatures ranging from about 350° C.to about 600° C.

In example operation 615, with reference also to FIG. 7B, asemiconductor nanowire stack 730 is formed on the base InAs layer 720through the mask layer 714. The nanowire stack 730 includes an InAsnanowire layer 732, a GaAs nanowire layer 734 and a GaSb nanowire layer736. The nanowire stack 730 may be formed either through a top-downapproach or through a bottom-up approach. For example in an bottom-upapproach, the nanowire InAs layer 732 is grown via selective area growth(SAG) approaches using MOCVD, vapor-phase epitaxy and/or crystalfacet-controlled epitaxial lateral overgrowth (FACELO) techniques orother suitable growth mechanisms in an aperture opened through the masklayer 714. Besides that the mask layer 714 serves the purposes ofselective area growth of nanowire InAs layer 732 through an apertureformed in the mask layer 714, other selective area growth techniques,e.g., by using a template hard mask, is also possible and included inthe disclosure. The nanowire InAs layer 732 is doped with the firstconductivity type as the base layer 720, here e.g., N-type.

An intrinsic or unintentionally doped epitaxy layer of GaAs is formedover the nanowire InAs 732 through MOCVD or MBE. A P-doped (P⁺) epitaxylayer of GaSb is formed over the epitaxy GaAs layer. The epitaxy GaAslayer and the epitaxy GaSb layer are patterned to form the nanowire GaAslayer 734 and the nanowire GaSb layer 736. The mask layer 714 serves asan etch stop layer in the epitaxy layer patterning. The patterning maybe implemented through a selective etch process, for example by KOHsolution. Another etching example includes a photo-electrochemical (PEC)treatment and a post-processing wet etch. The PEC treatment converts theunwanted portions to oxide. The oxide portions (side portions) are thenremoved by wet etching in buffered hydrofluoric acid (HF) and post-etchimmersion in potassium hydroxide (KOH) (0.5 M) at about 150° C. Othersuitable patterning approaches are also possible and included in thedisclosure.

In example operation 620, with reference also to FIG. 7C, a shell-shapedlayer 738 of intrinsic or unintentionally doped InAs is formedsurrounding a sidewall 730S of the nanowire stack 730. Specifically, theshell-shaped layer 738 contacts both the N⁺ InAs nanowire layer 732 andthe P⁺ GaSb nanowire layer 736. The shell-shaped layer 738 is formedthrough an epitaxy process using MOCVD or MBE and with a thicknessranging from about 1.5 nm to about 4 nm.

In example operation 625, a shell-shaped layer 750 of intrinsic orunintentionally doped GaAs is formed surrounding the shell layer 738.The shell-shaped GaAs layer is further away from the sidewall 730S ofthe vertical stack 730 than the shell-shaped InAs layer 738. Theshell-shaped layers 738, 750 are each formed through an epitaxy processusing MOCVD or MBE and with a thickness ranging from about 1 nm to about4 nm.

In example operation 630, with reference also to FIG. 7D, a sacrificialdielectric layer 752 is formed surrounding the shell-shaped layer 750and a sacrificial gate structure 754 is formed and patterned over thesacrificial dielectric layer 752 and also surrounding the shell-shapedlayer 750. The sacrificial gate structure 754 is polysilicon or othersuitable materials for a replacement gate process. The sacrificial gatestructure 754 is further away from the sidewall 730S of the verticalstack 730 than the shell-shaped GaAs layer 750.

In example operation 635, with reference also to FIG. 7E, inter-leveldielectric (ILD) layer 770 is formed. The ILD layer 770 includes adifferent dielectric material from the sacrificial dielectric layer 752such that the sacrificial dielectric layer 752 can be selectivelyremoved with the ILD layer 770 remaining.

In example operation 640, with reference also to FIG. 7F, contactstructures 760 are formed contacting the P⁺ GaSb nanowire layer 736 andthe N⁺ InAs base layer 720, which is electrically the same layer as thenanowire N⁺ InAs layer 732. The contact structures 760 are one or moreof tungsten W, cobalt Co, copper Cu or other suitable conductivematerials.

In example operation 645, with reference also to FIG. 7G, a void 772 isformed by removing the sacrificial gate structure 754 and part of thesacrificial dielectric layer 752 through an aperture 772 formed throughthe ILD layer 770.

In example operation 650, with reference also to FIG. 7H, replacementgate structure 740 is formed within the void 774. The replacement gatestructure 740 includes a high-K gate dielectric layer 742 and a metalgate electrode 744. The high-K gate dielectric layer 742 surrounds theshell-shaped InAs layer 738 with the shell-shaped GaAs layer 750positioned in-between.

The N⁺ nanowire InAs layer 732, alone or together with the N⁺ base layer720, can be configured as a drain, the P⁺ GaSb nanowire layer 736 can beconfigured as a source, the nanowire GaAs layer 734 can be configured asa barrier layer, and the intrinsic or unintentionally doped shell-shapedInAs layer 738 can be configured as a channel layer. The intrinsic orunintentionally doped shell-shaped GaAs layer 750 can be configured asan interlayer that barriers the movement of the charge carrier to theinterface with the gate dielectric 742 under the TAT tunneling effect.

The nanowire source/drain layers 736, 732 each may include a thicknessranging from about 20 nm to about 50 nm. The nanowire barrier layer 734may include a thickness ranging from about 10 nm to about 50 nm.

High-K dielectric material of the gate dielectric 742 may be selectedfrom one or more of hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO),hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HMO),hafnium titanium oxide (HMO), hafnium zirconium oxide (HfZrO),combinations thereof, and/or other suitable materials ZrO₂, Al₂O₃, LaO,TiO, Ta₂O₅, Y₂O₃, STO, BTO, BaZrO, HfLaO.

The High-K gate dielectric layer 742 may be formed by atomic layerdeposition (ALD) or other suitable technique. In accordance withembodiments described herein, high-K gate dielectric layer 742 includesa thickness ranging from about 5 to about 25 angstrom (Å) or othersuitable thickness.

In an embodiment, the metal gate electrode 744 is tungsten (W) ortitanium nitride (TiN). Other suitable materials for metal gate layer300 may include ruthenium, palladium, platinum, tungsten, cobalt,nickel, and/or conductive metal oxides and other suitable P-type metalmaterials and may include hafnium (Hf), zirconium (Zr), titanium (Ti),tantalum (Ta), aluminum (Al), aluminides and/or conductive metalcarbides (e.g., hafnium carbide, zirconium carbide, titanium carbide,and aluminum carbide), and other suitable materials for N-type metalmaterials.

The metal gate electrode 744 may be formed through sputtering or atomiclayer deposition (ALD).

With the interlayer 750, the OFF state leakage under the TAT tunnelingeffect is substantively reduced while the ON state BTBT tunneling is notaffected because the interlayer 750 does not affect the BTBT tunneling.As such, the TFET devices of the disclosure achieve high ON current, lowleakage and improved subthreshold swing SS.

The structure and functions of the interlayer may be applied to alltypes of TFET devices, which are not limited by the specific exampleTFET structures illustratively shown in the disclosure. For example, theinterlayer may be applied in either N-type or P-type TFETs with lateralheterojunctions or vertical heterojunction and including lateraltunneling components and/or vertical tunneling components. Theinterlayer is positioned between a semiconductor layer where chargecarriers are trapped and a heterojunction interface with a gatedielectric layer. The interlayer includes a semiconductor material thathas a valence band offset with respect to the semiconductor layer wherethe charge carrier is trapped such that the trapped charge carriers needto travel more energetic and spatial distance before reaching theinterface with the gate dielectric layer from the semiconductor layerwhere they are initially trapped.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdescription. Those skilled in the art should appreciate that they mayreadily use the present description as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present description, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present description.

In general, in the following claims, the terms used should not beconstrued to limit the claims to the specific embodiments disclosed inthe specification and the claims, but should be construed to include allpossible embodiments along with the full scope of equivalents to whichsuch claims are entitled. Accordingly, the claims are not limited by thedisclosure.

The present disclosure may be further appreciated with the descriptionof the following embodiments:

In a structure embodiment, a structure includes a first semiconductorlayer doped with a first conductivity type, a second semiconductor layerdoped with a second conductivity type, a third semiconductor layercontacting both the first semiconductor layer and the secondsemiconductor layer, a gate structure adjacent to the thirdsemiconductor layer, and a fourth semiconductor layer between the thirdsemiconductor layer and the gate structure. The fourth semiconductorlayer has a semiconductor material different from the thirdsemiconductor layer. The second semiconductor layer is separated fromthe first semiconductor layer.

In a device embodiment, a device includes a base layer of a firstsemiconductor material, a first nanowire source/drain region of thefirst semiconductor material overlaying the base layer, a secondnanowire source/drain region overlaying the first nanowire source/drainregion, a channel region contacting both the first nanowire source/drainregion and the second nanowire source/drain region, a gate structurelaterally adjacent to the channel region, and an interlayer of a secondsemiconductor material laterally between the channel region and the gatestructure, the second semiconductor material being different from thefirst semiconductor material.

A method embodiment receives a wafer that includes a base layer of afirst III-V compound semiconductor material over a substrate. A verticalstack of nanowire structures is formed over the base layer. The verticalstack includes a first nanowire structure of the first III-V compoundsemiconductor material and a second nanowire structure of a second III-Vcompound semiconductor material overlaying the first nanowiresemiconductor structure. The first nanowire structure is doped as afirst electronic conductivity type, and the second nanowiresemiconductor structure is doped as a second electronic conductivitytype. A first shell-shaped semiconductor layer is formed directlycontacting and surrounding a sidewall of the vertical stack of nanowirestructures. A second shell-shaped semiconductor layer is formedsurrounding the first shell-shaped semiconductor layer. The secondshell-shaped semiconductor layer has a different semiconductor materialfrom the first shell-shaped semiconductor layer. The second shell-shapedsemiconductor is further away from the sidewall of the vertical stackthan the first shell-shaped semiconductor layer. A gate structure isformed surrounding the second shell-shaped semiconductor layer. The gatestructure is further away from the sidewall of the vertical stack thanthe second shell-shaped semiconductor layer.

1. A structure, comprising: a substrate; a first semiconductor layerover the substrate and doped with a first conductivity type; a secondsemiconductor layer over the first semiconductor layer and doped with asecond conductivity type; a third semiconductor layer positioned in avertical direction between the first semiconductor layer and the secondsemiconductor layer, the third semiconductor layer on a side surface ofthe first semiconductor layer; a fourth semiconductor layer in contactwith both the second semiconductor layer and the third semiconductorlayer in a lateral direction, the fourth semiconductor layer including awider bandgap than the third semiconductor layer; and a gate structureadjacent to the fourth semiconductor layer, the fourth semiconductorlayer between the third semiconductor layer and the gate structure. 2.The structure of claim 1, wherein the third semiconductor layer ispositioned laterally between the fourth semiconductor layer and thefirst semiconductor layer.
 3. The structure of claim 1, comprising abase layer below the first semiconductor layer.
 4. The structure ofclaim 3, wherein the base layer is doped with the first conductivitytype.
 5. The structure of claim 3, comprising a nucleation layervertically between the substrate and the base layer.
 6. The structure ofclaim 3, comprising a contact structure in contact with the base layer.7. The structure of claim 6, wherein the fourth semiconductor layeroverlaps the base layer, and the contact structure extends through thefourth semiconductor layer.
 8. The structure of claim 1, wherein asemiconductor material of the fourth semiconductor layer includes aconduction band offset with respect to a semiconductor material of thethird semiconductor layer.
 9. The structure of claim 1, wherein asemiconductor material of the fourth semiconductor layer includes awider bandgap than a semiconductor material of the third semiconductorlayer.
 10. The structure of claim 1, wherein a semiconductor material ofthe fourth semiconductor layer includes a conduction band offset withrespect to a semiconductor material of one or more of the firstsemiconductor layer or the second semiconductor layer.
 11. The structureof claim 1, wherein the first semiconductor layer and the secondsemiconductor layer are both nanowire structures.
 12. The structure ofclaim 3, wherein the third semiconductor layer is one or more of InN orInGaN, and the fourth semiconductor layer is AlN.
 13. The structure ofclaim 12, wherein the base layer is GaN.
 14. A structure, comprising: asubstrate; a first source or drain layer of P-doped GaN over thesubstrate; a channel layer of InN or InGaN over the first source ordrain layer, the channel layer on a sidewall surface of the first sourceor drain layer; a second source or drain layer of N-doped GaN over thechannel layer; an interlayer of AlN in contact with both the channellayer and the second source or drain layer in a lateral direction; and agate structure adjacent to the interlayer, the interlayer between thechannel layer and the gate structure.
 15. The structure of claim 14,wherein the channel layer is intrinsic or unintentionally doped.
 16. Thestructure of claim 14, wherein the channel layer is positioned laterallybetween the interlay and the first source or drain layer.
 17. Thestructure of claim 14, comprising a base layer of P-doped GaN below thefirst source or drain layer.
 18. A structure, comprising: a substrate; afirst source or drain layer of N-doped InAs over the substrate; achannel layer of InAs over the first source or drain layer, the channellayer on a sidewall surface of the first source or drain layer; a secondsource or drain layer of P-doped semiconductor material over the channellayer; an interlayer of GaAs in contact with both the channel layer andthe second source or drain layer in a lateral direction; and a gatestructure adjacent to the interlayer, the interlayer between the channellayer and the gate structure.
 19. The structure of claim 18, wherein thesecond source or drain layer is GaSb.
 20. The structure of claim 18,wherein the second source or drain layer is silicon.